Inflammation is a nonspecific response of tissues to diverse stimuli and/or insults associated with the release of various mediators that induce pain, fever, and general sense of illness. Such responses can be reduced by the administration of non-steroidal or steroidal anti-inflammatory drugs. These anti-inflammatory drugs are associated with various side effect. However, the combination of non-steroidal anti-inflammatory drugs (NSAIDs) with glucocorticoids provides a synergistic anti-inflammatory and pain relief with less side effects. In this study, we aimed to synthesize a novel twin-drug of Dexamethasone-Diclofenac formulated into polylactide (PLA) nanoparticles to improve their solubility and provide a sustained release system. The twin-drug was synthesized through an esterification reaction which was then encapsulated into PLA nanoparticles. The hydrolysis of the synthesized twin-drug and drug release from PLA nanoparticles were studied in vitro with and without esterase enzyme. The anti-inflammatory activity was studied in BALB/c mice. After the successful synthesis of the twin-drug, its water solubility was improved by its encapsulation into PLA nanoparticles with a loading capacity of 66%. The in vitro release showed a sustained release profile of the twin-drug up to 52 h. The esterase hydrolysis showed a rapid release with a maximum hydrolysis after 1.5 h. Moreover, the anti-inflammatory activity exhibited a synergistic effect with a higher percentage of inhibition for the TNF-α level in comparison to the parent drugs after 6 h treatment. In conclusion, the prepared nano twin-drug is a novel therapy that showed a sustained release profile with an excellent anti-inflammatory activity.